It all depends on your perspective. Yes, some elements of the author's life, personality and way of seeing things make their way into their writing. No, a work of fiction is not a biography of its author. In particular, authors tend to have written more than one book. They cannot all be the author's biographies, can they? Nor are their main characters all the same person wearing a different name (at least not when good writers are concerned).
An easy example: Erich Maria Remarque's All Quiet on the Western Front was inspired by his own experiences during WW1. The main character bears some resemblance to Remarque; for example, if I remember correctly, there's mention of him writing, or at least that he used to write. The sense of disconnect when coming home for a furlough after having been at the front is also from his real experience. But All Quiet on the Western Front is not an autobiographical work. The characters in the story are not carbon copies of people he knew, and, of course, he didn't die in that war.
The same war has left its mark on J.R.R. Tolkien's The Lord of the Rings; Frodo's struggle to return into the society he fought to save, Éowyn talking about how hard it is to be left behind when all the men have gone to fight - those are experiences Tolkien was familiar with. The second was his own, the first - he would have observed. But the War of the Ring is not WW1 in any way, nor is Frodo - Tolkien, though they share some similarities of character (such as the love of a simple country life).
If you wish, the author, like any man, must necessary have his understanding of the Truth - of Right and Wrong, of how things are, of how people are, etc. That truth would seep into everything that he writes, by virtue of it being the lens through which he perceives the world around him. Each story the author writes might explore a different aspect of that Truth, but the Truth remains. Over time, the way the author sees things might change, and then it is his new Truth that would be seeping through. That Truth would be at the heart of how his characters are shaped, how they see and understand the world around them. (They might not necessarily share the author's understanding, but they would in some way shed a light on it.)
For more specific story elements, the author would turn to things he had experienced, things he has learned, things he has read about (those too are way of experiencing things, indirectly). Those are the bricks of his imagination, from them he would construct the tower of his literary work. In those ways, characters would necessarily carry something of the author inside them.
If you wish, my character is not me. But I can only understand my character by putting myself in them - in their shoes, in their head, in their culture and upbringing and situation. In this way, my understanding of how my character would act is a reflection of me.
Furthermore, what I choose to write about, the situations my character must face, the obstacles they must overcome - those too are a reflection of me. A man who has not experienced WW1 couldn't have written All Quiet on the Western Front, nor would they have had any impetus to write such a thing. But the main character of this work is not "Remarque", a particular person, but an Everyman, as Remarque would have seen him.